The damaging effects of sunlight on skin are well documented. Contrary to what most people believe, it is not necessary that one sunbathe to suffer the ill-effects of excessive ultraviolet (UV) exposure. In fact, significant damage can be caused by routine day-to-day activities in the sunlight. Some scientists estimate that over 70 percent of the damage the sun inflicts on the average person's skin over a lifetime is the result of simply being outdoors or even sitting by a window.
The major short term hazard of prolonged exposure to sunlight is erythema (i.e. sunburn). The 290 to 320 nanometer wavelength ultraviolet radiation range, designated as the "UVB" wavelength range, tends to be the primary cause of erythema, although the 320 to 400 nanometer wavelength ultraviolet radiation range, designated as the "UVA" wavelength range, also produces erythema.
In addition to the short term hazard of erythema, there are also long term hazards associated with UV radiation exposure. One of these long term hazards is malignant changes in the skin surface. Numerous epidemiologic studies demonstrate a strong relationship between sunlight exposure and human skin cancer.
Another long term hazard of ultraviolet radiation is premature aging of the skin. This condition is characterized by wrinkling and yellowing of the skin, along with other physical changes such as cracking, telangiectasis (spider vessels), solar keratoses (growths), ecchymoses (subcutaneous hemorrhagic lesions), and loss of elasticity (sagging). The adverse effects associated with exposure to UVA and UVB wavelength radiation are more fully discussed in DeSimone, "Sunscreen and Suntan Products", Handbook of Nonprescription Drugs, 7th Ed., Chapter 26, pp. 499-511 (American Pharmaceutical Association, Washington, D.C.; 1982); Grove and Forbes, "A Method for Evaluating the Photoprotection Action of Sunscreen Agents Against UV-A Radiation", International Journal of Cosmetic Science, 4, pp. 15-24 (1982); and U.S. Pat. No. 4,387,089, DePolo, issued Jun. 7, 1983; which are incorporated herein by reference in their entirety. Hence, although the immediate effects of ultraviolet radiation may be cosmetically and socially gratifying, the long-term hazards are cumulative and potentially serious. Therefore, photoprotection against ultraviolet radiation is necessary.
The fact that these effects are taken seriously by the general public is suggested by considering the sun protection product market. This market has grown considerably in recent years and many new products are introduced each year. What used to be looked upon as a seasonal business is no longer such. Sun protection compounds are now included in a diversity of personal care products, particularly cosmetic-type products which are worn on a daily basis.
The most common agents for sun protection are sunscreens. These agents exert their effects through chemical means, i.e., they absorb ultraviolet radiation so that it cannot penetrate the skin. Sunscreens present the user with several problems. For example, they must be on the surface of the skin at the time of exposure to be effective. Sunscreens are preventative so one must anticipate being in the sun. To be most effective, sunscreens must be on the skin as a continuous uniform film. Delivering such a film to the uneven surface of the skin is very difficult. Also, chemical sunscreens tend to migrate on or penetrate through the skin, thereby reducing their protective effect and potentially causing adverse reactions such as burning and stinging of the skin and eyes, and other more serious allergic and toxic reactions. Therefore, a need exists to develop sun protection products which maintain their efficacy and which can be used on the sensitive facial area without causing adverse reactions.
The sunscreening agent 2-phenylbenzimidazole-5-sulfonic acid is a known sunscreening agent, and is listed as a Category I material in "Sunscreen Drug Products for Over-the-Counter Human Use; Tentative Final Monograph; Proposed Rule", Federal Register vol. 58, no. 90, 28194, May 12, 1993, which is incorporated by reference herein in its entirety. However, 2-phenylbenzimidazole-5-sulfonic acid is utilized as a neutralized, water-soluble salt, rather than as the free acid. See E. Merck Technical Pamphlet No. 03-304111, entitled "Eusolex.RTM. UV Filters For Cosmetics", Feb. 8, 1990, and Haarman & Reimer Technical Pamphlet No. R221e, entitled "Neo Heliopan Sunscreen Filters," Feb. 1993, which are both incorporated by reference herein in their entirety. Using this sunscreen as a neutralized, water-soluble salt has the disadvantage of yielding a nonsubstantive sunscreening composition which is easily removed by water and perspiration, thereby resulting in greatly diminished sunscreen protection.
Conversely, the free acid form of 2-phenylbenzimidazole-5-sulfonic is generally not utilized because of its low water-solubility and perceived formulation limitations and reduced sunscreening efficacy. In other words, it is generally believed that 2-phenylbenzimidazole-5-sulfonic acid is not suitable for formulation into a low pH composition because it would exist predominantly in the free acid form and would crystallize out of the formulation. However, there are many instances when a low pH sunscreen formulation is desired, as when formulating an acidic active such as salicylic acid or lactic acid into the product. Therefore, the need exists for developing low pH compositions which also provide adequate sun protection.
It has been found in the present invention that 2-phenylbenzimidazole-5-sulfonic acid can be formulated into aqueous compositions which have an acidic, i.e. low pH. It has been found that the neutral free acid form of the 2-phenylbenzimidazole-5-sulfonic acid, despite its low-water solubility, provides compositions having good sunscreening efficacy. These compositions also have the further advantage of not burning or stinging the skin and of having reduced eye irritation potential. Without being limited by theory, it is believed that the unneutralized free acid form of the sunscreen is less likely to penetrate through the skin or migrate into the eyes when applied upon the face. Also, the sunscreen, in its free acid form is believed to be more substantive because it is less likely to be removed by moisture. Therefore, the compositions of the present invention which utilize the unneutralized, free acid form of 2-phenylbenzimidazole-5-sulfonic acid, provide benefits such as improved substantivity, less product migration, increased UV absorption, less burning, stinging, and other undesirable skin reactions, and better compatibility with acidic formulation components. Also, another benefit of utilizing the unneutralized 2-phenylbenzimidazole-5-sulfonic acid is that high SPF efficacy is achieved without the need for using high levels of additional sunscreen materials, which can negatively affect product aesthetics or increase product cost.
It is, therefore, an object of the present invention to provide topical sunscreen compositions which provide protection of human skin from the harmful effects of ultraviolet radiation which utilize the unneutralized free acid form of the sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid.
It is another object of the present invention to provide topical sunscreen compositions which have an acidic pH and which are compatible with other acidic components.
It is another object of the present invention to provide topical sunscreen compositions having enhanced substantivity, such as resistance to rub-off, water, and perspiration.
It is another object of the present invention to provide topical sunscreen compositions which are safe and nontoxic and which have less potential to burn or sting the skin or migrate into the eyes.
It is another object of the present invention to provide methods for protecting human skin from the harmful effects of ultraviolet radiation.
These and other objects will become readily apparent from the detailed description which follows.